Pathogen recognition receptors are important first responders to viral infection that bind to viral proteins or nucleic acids to trigger the immune response. RIG-1 is an RNA helicase with tandem caspase activation and recruitment domains (CARDs) that binds viral RNA and ultimately stimulates the genes induced by interferon regulator factor 3 (IRF-3) and nuclear factor κB (NF-κB). The CARDs of RIG-1 interact with the CARD of interferon (IFN)-β promoter stimulator 1 (IPS-1), an adaptor that links RIG-1 to the signaling pathway that leads to gene activation. Saito et al. report that RIG-1 is maintained inactive as a result of the interaction of a C-terminal repressor domain (RD), and this autoinhibition is alleviated by binding to RNA with a secondary structure, including RNA from hepatitis C virus. A related helicase, LGP2, which lacks CARDs, also bound double-stranded RNA, including sequences from hepatitis C virus. Mutation analysis showed that RNA binding by RIG-1 required the helicase domain and the C terminus but did not require the CARDs. Trypsin digestion studies indicated that upon binding RNA, RIG-1 underwent a conformational change that displaced the C-terminal region. Downstream signaling, the interaction with IPS-1, and gene activation by RIG-1 in response to Sendai virus infection required both CARDs. However, a RIG-1 with a C-terminal deletion was constitutively activated, and the C-terminal domain served as a dominant-negative inhibiting RIG-1. In transfected cells, Sendai virus induced the formation of RIG-1 multimers, and this was blocked by coexpression of the C-terminal RD. The C-terminal domain of LGP2 is similar to that of RIG-1, and when transfected into cells expressing RIG-1, LGP2 prevented induction of gene expression in response to viral infection. Full-length LGP2 or the C-terminal RD region formed a complex with RIG-1 when the proteins were overexpressed. Expression of the RIG-1 RD in cells conferred increased permissiveness to viral infection with Sendai virus. Thus, the RD of RIG-1 keeps RIG-1 inactive until viral RNA is present; then a conformational change alleviates this repression, allowing RIG-1 to interact with IPS-1 and stimulate genes involved in the immune response. LGP2, through its RD, appears to act in trans to inhibit RIG-1. Thus, these two proteins appear to serve as modulators of the response--RIG-1 turns it on, and LGP2 turns it off.